Brown dwarf

Brown dwarfsaresubstellar objectsthat have more mass than the biggestgas giantplanets, but less than the least massivemain-sequencestars.Their mass is approximately 13 to 80times that of Jupiter(M_J)—not big enough to sustainnuclear fusionof ordinaryhydrogen(¹H) into helium in their cores, but massive enough to emit some light and heat from thefusion of deuterium(²H). The most massive ones (>65M_J) canfuse lithium(⁷Li).

Artist's concept of a T-type brown dwarf

Comparison: most brown dwarfs are slightly larger in volume than Jupiter (15–20%),but are still up to 80 times more massive due to greater density. Image is to scale, with Jupiter's radius being 11 times that of Earth, and the Sun's radius 10 times that of Jupiter.

Astronomers classify self-luminous objects byspectral type,a distinction intimately tied to the surface temperature, and brown dwarfs occupy types M, L, T, and Y.As brown dwarfs do not undergo stable hydrogen fusion, they cool down over time, progressively passing through later spectral types as they age.

Their name comes not from the color of light they emit but from their falling betweenwhite dwarfstars and "dark" planets in size. To the naked eye, brown dwarfs would appear in different colors depending on their temperature.The warmest ones are possibly orange or red,while cooler brown dwarfs would likely appearmagentaor black to the human eye.Brown dwarfs may be fullyconvective,with no layers or chemical differentiation by depth.

Though their existence was initially theorized in the 1960s, it was not until the mid-1990s that the first unambiguous brown dwarfs were discovered. As brown dwarfs have relatively low surface temperatures, they are not very bright at visible wavelengths, emitting most of their light in theinfrared.However, with the advent of more capable infrared detecting devices, thousands of brown dwarfs have been identified. The nearest known brown dwarfs are located in theLuhman 16system, abinaryof L- and T-type brown dwarfs about 6.5light-years(2.0parsecs) from the Sun. Luhman 16 is the third closest system to the Sun afterAlpha CentauriandBarnard's Star.

History

The smaller object is Gliese 229B, about 20 to 50 times the mass of Jupiter, orbiting the starGliese 229.It is in the constellationLepus,about 19 light-years from Earth.

Early theorizing

Planets, brown dwarfs, stars (not to scale)

The objects now called "brown dwarfs" were theorized by Shiv S. Kumar in the 1960s to exist and were originally calledblack dwarfs,a classification for dark substellar objects floating freely in space that were not massive enough to sustain hydrogen fusion. However, (a) the term black dwarf was already in use to refer to a coldwhite dwarf;(b)red dwarfsfuse hydrogen; and (c) these objects may be luminous at visible wavelengths early in their lives. Because of this, alternative names for these objects were proposed, including planetar andsubstar.In 1975,Jill Tartersuggested the term "brown dwarf", using "brown" as an approximate color.

The term "black dwarf" still refers to awhite dwarfthat has cooled to the point that it no longer emits significant amounts of light. However, the time required for even the lowest-mass white dwarfto cool to this temperatureis calculated to be longer than the current age of the universe; hence such objects are expected to not yet exist.

Early theories concerning the nature of the lowest-mass stars and thehydrogen-burning limitsuggested that apopulation Iobject with a mass less than 0.07solar masses(M_☉) or apopulation IIobject less than 0.09M_☉would never go through normalstellar evolutionand would become a completelydegenerate star.The first self-consistent calculation of the hydrogen-burning minimum mass confirmed a value between 0.07 and 0.08 solar masses for population I objects.

Deuterium fusion

The discovery ofdeuterium burningdown to 0.013M_☉(13.6M_J) and the impact of dust formation in the cool outeratmospheresof brown dwarfs in the late 1980s brought these theories into question. However, such objects were hard to find because they emit almost no visible light. Their strongest emissions are in theinfrared(IR) spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs.

Since then, numerous searches by various methods have sought these objects. These methods included multi-color imaging surveys around field stars, imaging surveys for faint companions ofmain-sequencedwarfs andwhite dwarfs,surveys of youngstar clusters,andradial velocitymonitoring for close companions.

GD 165B and class L

For many years, efforts to discover brown dwarfs were fruitless. In 1988, however, a faint companion to the white dwarf starGD 165was found in an infrared search of white dwarfs. The spectrum of the companion GD 165B was very red and Enigma tic, showing none of the features expected of a low-massred dwarf.It became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until the advent of the Two Micron All-Sky Survey (2MASS) in 1997, which discovered many objects with similar colors and spectral features.

Today, GD 165B is recognized as the prototype of a class of objects now called "L dwarfs".

Although the discovery of the coolest dwarf was highly significant at the time, it was debated whether GD 165B would be classified as a brown dwarf or simply a very-low-mass star, because observationally it is very difficult to distinguish between the two.

Soon after the discovery of GD 165B, other brown-dwarf candidates were reported. Most failed to live up to their candidacy, however, because the absence of lithium showed them to be stellar objects. True starsburn their lithiumwithin a little over 100Myr,whereas brown dwarfs (which can, confusingly, have temperatures and luminosities similar to true stars) will not. Hence, the detection of lithium in the atmosphere of an object older than 100 Myr ensures that it is a brown dwarf.

Gliese 229B and class T

The first class "T" brown dwarf was discovered in 1994 byCaltechastronomersShrinivas Kulkarni,Tadashi Nakajima, Keith Matthews andRebecca Oppenheimer,andJohns HopkinsscientistsSamuel T. Durranceand David Golimowski. It was confirmed in 1995 as asubstellar companiontoGliese 229.Gliese 229b is one of the first two instances of clear evidence for a brown dwarf, along withTeide 1.Confirmed in 1995, both were identified by the presence of the 670.8 nm lithium line. The latter was found to have a temperature and luminosity well below the stellar range.

Its near-infrared spectrum clearly exhibited a methaneabsorption bandat 2 micrometres, a feature that had previously only been observed in the atmospheres of giant planets and that ofSaturn's moonTitan.Methane absorption is not expected at any temperature of a main-sequence star. This discovery helped to establish yet another spectral class even cooler thanLdwarfs, known as "Tdwarfs ", for which Gliese 229B is the prototype.

Teide 1 and class M

The first confirmed class "M" brown dwarf was discovered by Spanish astrophysicistsRafael Rebolo(head of the team), María Rosa Zapatero-Osorio, and Eduardo L. Martín in 1994.This object, found in thePleiadesopen cluster, received the nameTeide 1.The discovery article was submitted toNaturein May 1995, and published on 14 September 1995.Naturehighlighted "Brown dwarfs discovered, official" on the front page of that issue.

Teide 1was discovered in images collected by theIACteam on 6 January 1994 using the 80 cm telescope (IAC 80) atTeide Observatory,and its spectrum was first recorded in December 1994 using the 4.2 m William Herschel Telescope atRoque de los Muchachos Observatory(La Palma). The distance, chemical composition, and age of Teide 1 could be established because of its membership in the young Pleiades star cluster. Using the most advanced stellar and substellar evolution models at that moment, the team estimated for Teide 1 a mass of55 ± 15M_J,which is below the stellar-mass limit. The object became a reference in subsequent young brown dwarf related works.

In theory, a brown dwarf below65M_Jis unable to burn lithium by thermonuclear fusion at any time during its evolution. This fact is one of thelithiumtest principles used to judge the substellar nature of low-luminosity and low-surface-temperature astronomical bodies.

High-quality spectral data acquired by theKeck 1 telescopein November 1995 showed that Teide 1 still had the initial lithium abundance of the original molecular cloud from which Pleiades stars formed, proving the lack of thermonuclear fusion in its core. These observations confirmed that Teide 1 is a brown dwarf, as well as the efficiency of the spectroscopiclithium test.

For some time, Teide 1 was the smallest known object outside the Solar System that had been identified by direct observation. Since then, over 1,800 brown dwarfs have been identified,even some very close to Earth, likeEpsilon IndiBa and Bb, a pair of brown dwarfs gravitationally bound to a Sun-like star 12 light-years from the Sun,and Luhman 16, a binary system of brown dwarfs at 6.5 light-years from the Sun.

Theory

Hertzsprung–Russell diagram

Spectral type

O

B

A

F

G

K

M

L

T

Brown dwarfs

White dwarfs

Red dwarfs

Subdwarfs

Main sequence
( "dwarfs" )

Subgiants

Giants

Red giants

Blue giants

Bright giants

Supergiants

Red supergiant

Hypergiants

absolute
magni-
tude
(M_V)

The standard mechanism forstar birthis through the gravitational collapse of a cold interstellar cloud of gas and dust. As the cloud contracts, it heats due to theKelvin–Helmholtz mechanism.Early in the process the contracting gas quickly radiates away much of the energy, allowing the collapse to continue. Eventually, the central region becomes sufficiently dense to trap radiation. Consequently, the central temperature and density of the collapsed cloud increase dramatically with time, slowing the contraction, until the conditions are hot and dense enough for thermonuclear reactions to occur in the core of theprotostar.For a typical star, gas and radiation pressure generated by thethermonuclear fusionreactions within its core will support it against any further gravitational contraction.Hydrostatic equilibriumis reached, and the star will spend most of its lifetime fusing hydrogen into helium as a main-sequence star.

If, however, the initialmass of the protostar is less than about 0.08M_☉,normal hydrogenthermonuclear fusionreactions will not ignite in the core. Gravitational contraction does not heat the smallprotostarvery effectively, and before the temperature in the core can increase enough to trigger fusion, the density reaches the point where electrons become closely packed enough to create quantumelectron degeneracy pressure.According to the brown dwarf interior models, typical conditions in the core for density, temperature and pressure are expected to be the following:

• $10\,\mathrm {g/cm^{3}} \,\lesssim \,\rho _{c}\,\lesssim \,10^{3}\,\mathrm {{g}/{cm^{3}}}$
• $T_{c}\lesssim 3\times 10^{6}\,\mathrm {K}$
• $P_{c}\sim 10^{5}\,\mathrm {Mbar}.$

This means that the protostar is not massive or dense enough ever to reach the conditions needed to sustain hydrogen fusion. The infalling matter is prevented, by electron degeneracy pressure, from reaching the densities and pressures needed.

Further gravitational contraction is prevented and the result is a brown dwarf that simply cools off by radiating away its internal thermal energy. Note that, in principle, it is possible for a brown dwarf to slowly accrete mass above thehydrogen burning limitwithout initiating hydrogen fusion. This could happen via mass transfer in a binary brown dwarf system.

High-mass brown dwarfs versus low-mass stars

Lithiumis generally present in brown dwarfs and not in low-mass stars. Stars, which reach the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. Fusion oflithium-7and aprotonoccurs, producing twohelium-4nuclei. The temperature necessary for this reaction is just below that necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is eventually depleted. Therefore, the presence of the lithium spectral line in a candidate brown dwarf is a strong indicator that it is indeed a substellar object.

The lithium test

The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as thelithium test,and was pioneered byRafael Rebolo,Eduardo MartínandAntonio Magazzu.However, lithium is also seen in very young stars, which have not yet had enough time to burn it all.

Heavier stars, like the Sun, can also retain lithium in their outer layers, which never get hot enough to fuse lithium, and whose convective layer does not mix with the core where the lithium would be rapidly depleted. Those larger stars are easily distinguishable from brown dwarfs by their size and luminosity.

Conversely, brown dwarfs at the high end of their mass range can be hot enough to deplete their lithium when they are young. Dwarfs of mass greater than65M_Jcan burn their lithium by the time they are half a billion years old;thus the lithium test is not perfect.

Atmospheric methane

Unlike stars, older brown dwarfs are sometimes cool enough that, over very long periods of time, their atmospheres can gather observable quantities ofmethane,which cannot form in hotter objects. Dwarfs confirmed in this fashion includeGliese 229B.

Iron, silicate and sulfide clouds

Main-sequence stars cool, but eventually reach a minimumbolometric luminositythat they can sustain through steady fusion. This luminosity varies from star to star, but is generally at least 0.01% that of the Sun.Brown dwarfs cool and darken steadily over their lifetimes; sufficiently old brown dwarfs will be too faint to be detectable.

Cloud models for the early T-type brown dwarfsSIMP J0136+09and2MASS J2139+02(left two panels) and the late T-type brown dwarf 2M0050–3322.

Clouds are used to explain the weakening of theiron hydride(FeH) spectral line in late L-dwarfs.Ironclouds deplete FeH in the upper atmosphere, and the cloud layer blocks the view to lower layers still containing FeH. The later strengthening of this chemical compound at cooler temperatures of mid- to late T-dwarfs is explained by disturbed clouds that allows a telescope to look into the deeper layers of the atmosphere that still contains FeH.Young L/T-dwarfs (L2-T4) show highvariability,which could be explained with clouds, hot spots, magnetically drivenauroraeorthermochemicalinstabilities.The clouds of these brown dwarfs are explained as either iron clouds with varying thickness or a lower thick iron cloud layer and an uppersilicatecloud layer. This upper silicate cloud layer can consist out ofquartz,enstatite,corundumand/orfosterite.It is however not clear if silicate clouds are always necessary for young objects.Silicate absorption can be directly observed in themid-infraredat 8 to 12 μm. Observations withSpitzer IRShave shown that silicate absorption is common, but not ubiquitous, for L2-L8 dwarfs.Additionally,MIRIhas observed silicate absorption in the planetary-mass companionVHS 1256b.

Iron rainas part of atmospheric convection processes is possible only in brown dwarfs, and not in small stars. The spectroscopy research into iron rain is still ongoing, but not all brown dwarfs will always have this atmospheric anomaly. In 2013, a heterogeneous iron-containing atmosphere was imaged around the B component in the nearby Luhman 16 system.

For late T-type brown dwarfs only a few variable searches were carried out. Thin cloud layers are predicted to form in late T-dwarfs fromchromiumandpotassium chloride,as well as severalsulfides.These sulfides aremanganese sulfide,sodium sulfideandzinc sulfide.The variable T7 dwarf2M0050–3322is explained to have a top layer of potassium chloride clouds, a mid layer of sodium sulfide clouds and a lower layer of manganese sulfide clouds. Patchy clouds of the top two cloud layers could explain why the methane and water vapor bands are variable.

At the lowest temperatures of the Y-dwarfWISE 0855-0714patchy cloud layers of sulfide andwater iceclouds could cover 50% of the surface.

Low-mass brown dwarfs versus high-mass planets

An artistic concept of the brown dwarf around the starHD 29587,a companion known asHD 29587 b,estimated to be about 55 Jupiter masses

Like stars, brown dwarfs form independently, but, unlike stars, they lack sufficient mass to "ignite" hydrogen fusion. Like all stars, they can occur singly or in close proximity to other stars. Some orbit stars and can, like planets, have eccentric orbits.

Size and fuel-burning ambiguities

Brown dwarfs are all roughly the same radius as Jupiter. At the high end of their mass range (60–90M_J), the volume of a brown dwarf is governed primarily byelectron-degeneracypressure,as it is in white dwarfs; at the low end of the range (10M_J), their volume is governed primarily byCoulomb pressure,as it is in planets. The net result is that the radii of brown dwarfs vary by only 10–15% over the range of possible masses. Moreover, the mass–radius relationship shows no change from about one Saturn mass to the onset of hydrogen burning (0.080±0.008M_☉), suggesting that from this perspective brown dwarfs are simply high-mass Jovian planets.This can make distinguishing them from planets difficult.

In addition, many brown dwarfs undergo no fusion; even those at the high end of the mass range (over60M_J) cool quickly enough that after 10 million years they no longer undergofusion.

Heat spectrum

X-ray and infrared spectra are telltale signs of brown dwarfs. Some emitX-rays;and all "warm" dwarfs continue to glow tellingly in the red andinfraredspectra until they cool to planet-like temperatures (under 1,000 K).

Gas giantshave some of the characteristics of brown dwarfs. Like the Sun,JupiterandSaturnare both made primarily of hydrogen and helium. Saturn is nearly as large as Jupiter, despite having only 30% the mass. Three of the giant planets in the Solar System (Jupiter, Saturn, andNeptune) emit much more (up to about twice) heat than they receive from the Sun.All four giant planets have their own "planetary" systems, in the form of extensive moon systems.

Current IAU standard

Currently, theInternational Astronomical Unionconsiders an object above13M_J(the limiting mass for thermonuclear fusion of deuterium) to be a brown dwarf, whereas an object under that mass (and orbiting a star or stellar remnant) is considered a planet. The minimum mass required to trigger sustained hydrogen burning (about80M_J) forms the upper limit of the definition.

It is also debated whether brown dwarfs would be better defined by their formation process rather than by theoretical mass limits based on nuclear fusion reactions.Under this interpretation brown dwarfs are those objects that represent the lowest-mass products of thestar formationprocess, while planets are objects formed in anaccretion disksurrounding a star. The coolest free-floating objects discovered, such asWISE 0855,as well as the lowest-mass young objects known, likePSO J318.5−22,are thought to have masses below13M_J,and as a result are sometimes referred to asplanetary-mass objectsdue to the ambiguity of whether they should be regarded asrogue planetsor brown dwarfs. There are planetary-mass objects known to orbit brown dwarfs, such as2M1207b,MOA-2007-BLG-192Lb,2MASS J044144band Oph 98 B.

The 13-Jupiter-mass cutoff is a rule of thumb rather than a quantity with precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the 13‑Jupiter-mass value is somewhere in between.The amount of deuterium burnt also depends to some extent on the composition of the object, specifically on the amount ofheliumanddeuteriumpresent and on the fraction of heavier elements, which determines the atmospheric opacity and thus the radiative cooling rate.

As of 2011 theExtrasolar Planets Encyclopaediaincluded objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around13M_Jupin the observed mass spectrum reinforces the choice to forget this mass limit ".As of 2016, this limit was increased to 60 Jupiter masses,based on a study of mass–density relationships.

TheExoplanet Data Explorerincludes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to thesin i ambiguity."TheNASA Exoplanet Archiveincludes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.

Sub-brown dwarf

Main article:Sub-brown dwarf

A size comparison between theSun,a young sub-brown dwarf, andJupiter.As the sub-brown dwarf ages, it will gradually cool and shrink.

Objects below13M_J,calledsub-brown dwarfsorplanetary-mass brown dwarfs,form in the same manner asstarsand brown dwarfs (i.e. through the collapse of agas cloud) but have amass below the limiting mass for thermonuclear fusionofdeuterium.

Some researchers call them free-floating planets,whereas others call them planetary-mass brown dwarfs.

Role of other physical properties in the mass estimate

While spectroscopic features can help to distinguish betweenlow-mass starsand brown dwarfs, it is often necessary to estimate the mass to come to a conclusion. The theory behind the mass estimate is that brown dwarfs with a similar mass form in a similar way and are hot when they form. Some have spectral types that are similar to low-mass stars, such as2M1101AB.As they cool down the brown dwarfs should retain a range ofluminositiesdepending on the mass.Without the age and luminosity, a mass estimate is difficult; for example, an L-type brown dwarf could be an old brown dwarf with a high mass (possibly a low-mass star) or a young brown dwarf with a very low mass. For Y dwarfs this is less of a problem, as they remain low-mass objects near thesub-brown dwarflimit, even for relatively high age estimates.For L and T dwarfs it is still useful to have an accurate age estimate. The luminosity is here the less concerning property, as this can be estimated from thespectral energy distribution.The age estimate can be done in two ways. Either the brown dwarf is young and still has spectral features that are associated with youth, or the brown dwarf co-moves with a star or stellar group (star clusterorassociation), where age estimates are easier to obtain. A very young brown dwarf that was further studied with this method is2M1207and the companion2M1207b.Based on the location,proper motionand spectral signature, this object was determined to belong to the ~8-million-year-oldTW Hydrae association,and the mass of the secondary was determined to be 8 ± 2M_J,below thedeuteriumburning limit.An example of a very old age obtained by the co-movement method is the brown dwarf +white dwarfbinary COCONUTS-1, with the white dwarf estimated to be7.3+2.8
−1.6billion yearsold. In this case the mass was not estimated with the derived age, but the co-movement provided an accurate distance estimate, usingGaiaparallax.Using this measurement the authors estimated the radius, which was then used to estimate the mass for the brown dwarf as15.4+0.9
−0.8M_J.

Observations

Classification of brown dwarfs

Spectral class M

Artist's vision of a late-M dwarf

These are brown dwarfs with a spectral class of M5.5 or later; they are also called late-M dwarfs. Some scientists regard them asred dwarfs.All brown dwarfs with spectral type M are young objects, such asTeide 1,which is the first M-type brown dwarf discovered, andLP 944-20,the closest M-type brown dwarf.

Spectral class L

Artist's concept of an L dwarf

The defining characteristic ofspectral classM, the coolest type in the long-standing classical stellar sequence, is an optical spectrum dominated by absorption bands oftitanium(II) oxide(TiO) andvanadium(II) oxide(VO) molecules. However,GD 165B, the cool companion to the white dwarfGD 165,had none of the hallmark TiO features of M dwarfs. The subsequent identification of many objects like GD 165B ultimately led to the definition of a newspectral class,theL dwarfs,defined in the red optical region of the spectrum not by metal-oxide absorption bands (TiO, VO), but by metalhydrideemission bands (FeH,CrH,MgH,CaH) and prominent atomic lines ofalkali metals(Na, K, Rb, Cs). As of 2013,over 900 L dwarfs had been identified,most by wide-field surveys: the Two Micron All Sky Survey (2MASS), theDeep Near Infrared Survey of the Southern Sky(DENIS), and theSloan Digital Sky Survey(SDSS). This spectral class also contains the coolest main-sequence stars (> 80 M_J), which have spectral classes L2 to L6.

Spectral class T

Artist's concept of a T dwarf

As GD 165B is the prototype of the L dwarfs,Gliese 229B is the prototype of a second new spectral class, theT dwarfs.T dwarfs are pinkish-magenta. Whereasnear-infrared(NIR) spectra of L dwarfs show strong absorption bands of H₂O andcarbon monoxide(CO), the NIR spectrum of Gliese 229B is dominated by absorption bands frommethane(CH₄), a feature which in the Solar System is found only in the giant planets andTitan.CH₄,H₂O, and molecularhydrogen(H₂) collision-induced absorption (CIA) give Gliese 229B blue near-infrared colors. Its steeply sloped red optical spectrum also lacks the FeH and CrH bands that characterize L dwarfs and instead is influenced by exceptionally broad absorption features from thealkalimetalsNaandK.These differences ledJ. Davy Kirkpatrickto propose the T spectral class for objects exhibiting H- and K-band CH₄absorption. As of 2013,355 T dwarfs were known.NIR classification schemes for T dwarfs have recently been developed by Adam Burgasser and Tom Geballe. Theory suggests that L dwarfs are a mixture of very-low-mass stars and sub-stellar objects (brown dwarfs), whereas the T dwarf class is composed entirely of brown dwarfs. Because of the absorption ofsodiumandpotassiumin the green part of the spectrum of T dwarfs, the actual appearance of T dwarfs to humanvisual perceptionis estimated to be not brown, butmagenta.T-class brown dwarfs, such asWISE 0316+4307,have been detected more than 100 light-years from the Sun.

Spectral class Y

See also:List of Y-dwarfs

Artist's vision of a Y dwarf

In 2009, the coolest-known brown dwarfs had estimated effective temperatures between 500 and 600K(227–327°C;440–620°F), and have been assigned the spectral class T9. Three examples are the brown dwarfsCFBDS J005910.90–011401.3,ULAS J133553.45+113005.2andULAS J003402.77−005206.7.The spectra of these objects have absorption peaks around 1.55 micrometres.Delorme et al. have suggested that this feature is due to absorption fromammoniaand that this should be taken as indicating the T–Y transition, making these objects of type Y0.However, the feature is difficult to distinguish from absorption by water andmethane,and other authors have stated that the assignment of class Y0 is premature.

The firstJWSTspectral energy distribution of a Y-dwarf was able to observe several bands of molecules in the atmosphere of the Y0-dwarfWISE 0359−5401.The observations covered spectroscopy from 1 to 12 μm and photometry at 15, 18 and 21 μm. The molecules water (H₂O), methane (CH₄), carbon monoxide (CO), carbon dioxide (CO₂) and ammonia (NH₃) were detected in WISE 0359−5401. Many of these features have been observed before in this Y-dwarf and warmer T-dwarfs by other observatories, but JWST was able to observe them in a single spectrum. Methane is the main reservoir of carbon in the atmosphere of WISE 0359−5401, but there is still enough carbon left to form detectable carbon monoxide (at 4.5–5.0 μm) and carbon dioxide (at 4.2–4.35 μm) in the Y-dwarf. Ammonia was difficult to detect before JWST, as it blends in with the absorption feature of water in the near-infrared, as well at 5.5–7.1 μm. At longer wavelengths of 8.5–12 μm the spectrum of WISE 0359−5401 is dominated by the absorption of ammonia. At 3 μm there is an additional newly detected ammonia feature.

Colder lower atmosphere

Usually brown dwarfs have apressure–temperature(P–T) profile in anadiabaticform, which means that the pressure and temperature increase with depth. JWST spectroscopy andphotometrysuggest that Y-dwarfs have P–T profiles that are not in the standard adiabatic form. This means that upper layers of the atmosphere have a warmer temperature and lower layers of the atmosphere have a colder temperature. This is explained with the rapidrotationof these isolated objects. The rapid rotation leads to dynamical, thermal, and chemical changes, which disrupt theconvectivetransport of heat from the lower to the upper atmosphere. This different P–T profile influences the shape of the spectrum and influences the composition of carbon- and nitrogen-bearing molecules in the atmosphere of Y-dwarfs.

Individual Y-dwarf discoveries

Timeline of Y-dwarf discoveries:

• April 2010: Two newly discovered ultracoolsub-brown dwarfs(UGPS 0722-05and SDWFS 1433+35) were proposed as prototypes for spectral class Y0.

• February 2011: Luhman et al. reported the discovery ofWD 0806−661 B,a brown dwarf companion to a nearby white dwarf, with a temperature of c. 300 K (27 °C; 80 °F) and mass of7M_J.Though of planetary mass, Rodriguez et al. suggest it is unlikely to have formed in the same manner as planets.

• February 2011: Shortly after that, Liu et al. published an account of a "very cold" (c. 370 K (97 °C; 206 °F)) brown dwarf orbiting another very-low-mass brown dwarf and noted, "Given its low luminosity, atypical colors and cold temperature,CFBDS J1458+10Bis a promising candidate for the hypothesized Y spectral class. "

• August 2011: Scientists using data from NASA'sWide-field Infrared Survey Explorer(WISE) discovered six objects that they classified as Y dwarfs with temperatures as cool as 25 °C (298 K; 77 °F).These were published in two papers.

WISE 0458+6434is the first ultra-cool brown dwarf (green dot) discovered byWISE.The green and blue comes from infrared wavelengths mapped to visible colors.

• July 2012: Seven new Y-dwarfs were discovered, making the total number of confirmed Y-dwarfs fourteen.One of the Y dwarfs, calledWISE 1828+2650,was, as of August 2011, the record holder for the coldest brown dwarf—emitting no visible light at all, this type of object resembles a free-floating planet more than a star. WISE 1828+2650 was initially estimated to have an atmospheric temperature cooler than 300 K (27 °C; 80 °F).Its temperature has since been revised, and newer estimates put it in the range of 250 to 400 K (−23 to 127 °C; −10 to 260 °F).

• November 2012:WISE J1639−6847was discovered. As of February 2024, it was the second-closest known Y-dwarf to Earth.
• April 2014:WISE 0855−0714was announced, with a temperature profile estimated around 225 to 260 K (−48 – −13 °C; −55–8 °F) and a mass of3 to 10M_J.It was also unusual in that its observed parallax meant a distance close to7.2 ± 0.7light-years from the Solar System.
• May 2014: The Y-dwarfWISE J2209+2711was published.
• November 2014: The objectWISEA J1141−3326was estimated to be a Y-dwarfand it was later confirmed.
• April 2015: The T+Y dwarf binaryWISE J0146+4234 ABwas discovered.
• May 2015: Three Y-dwarfs were discovered with Hubble, bringing the total number of confirmed Y-dwarfs to 21.
• June 2018:WISEA J0302−5817was published as a Y-dwarf, and WISEA J1141−3326 was confirmed as a Y-dwarf.
• August 2019: A search of the CatWISE catalog revealedCWISEP J1935-1546,one of the coldest brown dwarfs with an estimated temperature of 270 to 360 K (−3–87 °C; 26–188 °F).In 2023 it was announced that CWISEP J1935-1546 had methane emission due to an aurora.
• January 2020: In January 2020 the discovery ofWISE J0830+2837,initially discovered bycitizen scientistsof theBackyard Worldsproject, was presented at the 235th meeting of theAmerican Astronomical Society.This Y dwarf is 36.5 light-years distant from the Solar System and has a temperature of about 350 K (77 °C; 170 °F).

• February 2020: The CatWISE catalog combined NASA's WISE andNEOWISEsurveys.It expanded the number of faint sources and has therefore been used to find the faintest brown dwarfs, including Y dwarfs. Seventeen candidate Y dwarfs were discovered by the CatWISE researchers. Initial color with theSpitzer Space Telescopeindicated thatCW1446is one of the reddest and coldest Y dwarfs.Additional data with Spitzer showed that CW1446 is the fifth-reddest brown dwarf, with a temperature of about 310 to 360 K (37–87 °C; 98–188 °F) and a distance of about 10 parsecs.
• August 2020: Five candidate Y-dwarfs were discovered via the Backyard Worlds project.
• April 2021: New Y-dwarf candidates were published by the CatWISE and Backyard Worlds teams in a collaborative paper.
• August 2021:Ross 19B,an old object near the T/Y-boundary orbiting an M-dwarf, was discovered by the Backyard Worlds team.
• April 2023:WISE J0336−0143was confirmed as a Y-dwarf binary with JWST.The B secondary is likely one of the coldest confirmed Y-dwarfs as of December 2023, with an estimated temperature of 246 to 404 K (−27–131 °C; −17–268 °F).
• November 2023:CWISE J1055+5443,an object previously classified as a T-dwarf, was confirmed as a nearby Y-dwarf.
• December 2023: Three new Y-dwarf candidates were published.The total number of confirmed Y-dwarfs was 27, and 30 additional Y-dwarf candidates existed as of February 2024.
• January 2024: Two candidate planets orbiting white dwarfs were discovered with JWST. If spectroscopically confirmed they would likely be Y-dwarfs due to their cold estimated temperature (T_eff<200 K).

Role of vertical mi xing

Major chemical pathways linking carbon monoxide and methane. The short-lived radicals are marked with a dot. Adopted from Zahnle & Marley

In the hydrogen-dominated atmosphere of brown dwarfs achemical equilibriumbetweencarbon monoxideandmethaneexists. Carbon monoxide reacts withhydrogenmolecules and forms methane andhydroxylin this reaction. The hydroxyl radical might later react with hydrogen and form water molecules. In the other direction of the reaction, methane reacts with hydroxyl and forms carbon monoxide and hydrogen. The chemical reaction is tilted towards carbon monoxide at higher temperatures (L-dwarfs) and lower pressure. At lower temperatures (T-dwarfs) and higher pressure the reaction is tilted towards methane, and methane predominates at the T/Y-boundary. However, vertical mi xing of the atmosphere can cause methane to sink into lower layers of the atmosphere and carbon monoxide to rise from these lower and hotter layers. The carbon monoxide is slow to react back into methane because of an energy barrier that prevents the breakdown of theC-O bonds.This forces the observable atmosphere of a brown dwarf to be in a chemical disequilibrium. The L/T transition is mainly defined with the transition from a carbon-monoxide-dominated atmosphere in L-dwarfs to a methane-dominated atmosphere in T-dwarfs. The amount of vertical mi xing can therefore push the L/T-transition to lower or higher temperatures. This becomes important for objects with modest surface gravity and extended atmospheres, such as giantexoplanets.This pushes the L/T transition to lower temperatures for giant exoplanets. For brown dwarfs this transition occurs at around 1200 K. The exoplanetHR 8799c,on the other hand, does not show any methane, while having a temperature of 1100K.

The transition between T- and Y-dwarfs is often defined as 500 K because of the lack of spectral observations of these cold and faint objects.Future observations withJWSTand theELTsmight improve the sample of Y-dwarfs with observed spectra. Y-dwarfs are dominated by deep spectral features of methane, water vapor and possibly absorption features ofammoniaandwater ice.Vertical mi xing, clouds, metallicity,photochemistry,lightning,impact shocks and metalliccatalystsmight influence the temperature at which the L/T and T/Y transition occurs.

Secondary features

Brown dwarf spectral types

Secondary features
pec	This suffix (e.g. L2pec) stands for "peculiar".
sd	This prefix (e.g. sdL0) stands forsubdwarfand indicates a low metallicity and blue color
β	Objects with the beta (β) suffix (e.g. L4β) have an intermediate surface gravity.
γ	Objects with the gamma (γ) suffix (e.g. L5γ) have a low surface gravity.
red	The red suffix (e.g. L0red) indicates objects without signs of youth, but high dust content
blue	The blue suffix (e.g. L3blue) indicates unusual blue near-infrared colors for L dwarfs without obvious low metallicity

Young brown dwarfs have lowsurface gravitiesbecause they have larger radii and lower masses than the field stars of similar spectral type. These sources are noted by a letterbeta(β) for intermediate surface gravity orgamma(γ) for low surface gravity. Indicators of low surface gravity include weak CaH, K I and Na I lines, as well as a strong VO line.Alpha(α) denotes normal surface gravity and is usually dropped. Sometimes an extremely low surface gravity is denoted by a delta (δ).The suffix "pec" stands for "peculiar"; this suffix is still used for other features that are unusual, and summarizes different properties, indicating low surface gravity, subdwarfs and unresolved binaries.The prefix sd stands forsubdwarfand only includes cool subdwarfs. This prefix indicates a lowmetallicityand kinematic properties that are more similar tohalostars than todiskstars.Subdwarfs appear bluer than disk objects.The red suffix describes objects with red color, but an older age. This is not interpreted as low surface gravity, but as a high dust content.The blue suffix describes objects with bluenear-infraredcolors that cannot be explained with low metallicity. Some are explained as L+T binaries, others are not binaries, such as2MASS J11263991−5003550and are explained with thin and/or large-grained clouds.

Spectral and atmospheric properties of brown dwarfs

Artist's illustration of a brown dwarf's interior structure. Cloud layers at certain depths are offset as a result of layer shifting.

The majority of flux emitted by L and T dwarfs is in the 1- to 2.5-micrometre near-infrared range. Low and decreasing temperatures through the late-M, -L, and -T dwarf sequence result in a rich near-infraredspectrumcontaining a wide variety of features, from relatively narrow lines of neutral atomic species to broad molecular bands, all of which have different dependencies on temperature, gravity, andmetallicity.Furthermore, these low temperature conditions favor condensation out of the gas state and the formation of grains.

Wind measured (Spitzer ST; Artist Concept; 9 Apr 2020)

Typical atmospheres of known brown dwarfs range in temperature from 2,200 down to750K.Compared to stars, which warm themselves with steady internal fusion, brown dwarfs cool quickly over time; more massive dwarfs cool more slowly than less massive ones. There is some evidence that the cooling of brown dwarfs slows down at the transition between spectral classes L and T (about 1000 K).

Observations of known brown dwarf candidates have revealed a pattern of brightening and dimming of infrared emissions that suggests relatively cool, opaque cloud patterns obscuring a hot interior that is stirred by extreme winds. The weather on such bodies is thought to be extremely strong, comparable to but far exceeding Jupiter's famous storms.

On January 8, 2013, astronomers using NASA'sHubbleandSpitzerspace telescopes probed the stormy atmosphere of a brown dwarf named2MASS J22282889–4310262,creating the most detailed "weather map" of a brown dwarf thus far. It shows wind-driven, planet-sized clouds. The new research is a stepping stone toward a better understanding not only brown dwarfs, but also of the atmospheres of planets beyond the Solar System.

In April 2020 scientists reported clocking wind speeds of +650 ± 310 metres per second (up to 1,450 miles per hour) on the nearby brown dwarf2MASS J10475385+2124234.To calculate the measurements, scientists compared the rotational movement of atmospheric features, as ascertained by brightness changes, against the electromagnetic rotation generated by the brown dwarf's interior. The results confirmed previous predictions that brown dwarfs would have high winds. Scientists are hopeful that this comparison method can be used to explore the atmospheric dynamics of other brown dwarfs and extrasolar planets.

Observational techniques

Brown dwarfsTeide 1,Gliese 229B, andWISE 1828+2650compared to red dwarfGliese 229A,Jupiter and our Sun

Coronagraphshave recently been used to detect faint objects orbiting bright visible stars, including Gliese 229B.

Sensitive telescopes equipped with charge-coupled devices (CCDs) have been used to search distant star clusters for faint objects, including Teide 1.

Wide-field searches have identified individual faint objects, such asKelu-1(30 light-years away).

Brown dwarfs are often discovered in surveys to discoverexoplanets.Methods of detecting exoplanetswork for brown dwarfs as well, although brown dwarfs are much easier to detect.

Brown dwarfs can be powerful emitters of radio emission due to their strong magnetic fields. Observing programs at theArecibo Observatoryand theVery Large Arrayhave detected over a dozen such objects, which are also calledultracool dwarfsbecause they share common magnetic properties with other objects in this class.The detection of radio emission from brown dwarfs permits their magnetic field strengths to be measured directly.

Milestones

• 1995: First brown dwarf verified.Teide 1,an M8 object in thePleiadescluster,is picked out with a CCD in the Spanish Observatory of Roque de los Muchachos of theInstituto de Astrofísica de Canarias.
• First methane brown dwarf verified. Gliese 229B is discovered orbiting red dwarfGliese 229A (20 ly away) using anadaptive opticscoronagraph to sharpen images from the 60-inch (1.5 m) reflecting telescope atPalomar Observatoryon Southern California'sMount Palomar;follow-up infrared spectroscopy made with their 200-inch (5.1 m)Hale Telescopeshows an abundance of methane.
• 1998: First X-ray-emitting brown dwarf found. Cha Helpha 1, an M8 object in theChamaeleon Idark cloud, is determined to be an X-ray source, similar to convective late-type stars.
• 15 December 1999: First X-ray flare detected from a brown dwarf. A team at the University of California monitoringLP 944-20(60M_J,16 ly away) via theChandra X-ray Observatory,catches a 2-hour flare.
• 27 July 2000: First radio emission (in flare and quiescence) detected from a brown dwarf. A team of students at theVery Large Arraydetected emission from LP 944–20.
• 30 April 2004: First detection of a candidateexoplanetaround a brown dwarf:2M1207bdiscovered with theVLTand the first directly imaged exoplanet.
• 20 March 2013: Discovery of the closest brown dwarf system: Luhman 16.
• 25 April 2014: Coldest-known brown dwarf discovered.WISE 0855−0714is 7.2 light-years away (seventh-closest system to the Sun) and has a temperature between −48 and −13 °C.

Brown dwarfs X-ray sources

Chandraimage ofLP 944-20before flare and during flare

X-ray flares detected from brown dwarfs since 1999 suggest changingmagnetic fieldswithin them, similar to those in very-low-mass stars. Although they do not fuse hydrogen into helium in their cores like stars, energy from the fusion of deuterium and gravitational contraction keep their interiors warm and generate strong magnetic fields. The interior of a brown dwarf is in a rapidly boiling, or convective state. When combined with the rapid rotation that most brown dwarfs exhibit,convectionsets up conditions for the development of a strong, tangledmagnetic fieldnear the surface. The magnetic fields that generated the flare observed byChandrafromLP 944-20has its origin in the turbulent magnetizedplasmabeneath the brown dwarf's "surface".

Using NASA'sChandra X-ray Observatory,scientists have detected X-rays from a low-mass brown dwarf in a multiple star system.This is the first time that a brown dwarf this close to its parent star(s) (Sun-like stars TWA 5A) has been resolved in X-rays."Our Chandra data show that the X-rays originate from the brown dwarf's coronal plasma which is some 3 million degrees Celsius", said Yohko Tsuboi ofChuo Universityin Tokyo."This brown dwarf is as bright as the Sun today in X-ray light, while it is fifty times less massive than the Sun", said Tsuboi."This observation, thus, raises the possibility that even massive planets might emit X-rays by themselves during their youth!"

Brown dwarfs as radio sources

The first brown dwarf that was discovered to emit radio signals wasLP 944-20,which was observed since it is also a source of X-ray emission, and both types of emission are signatures of coronae. Approximately 5–10% of brown dwarfs appear to have strong magnetic fields and emit radio waves, and there may be as many as 40 magnetic brown dwarfs within 25 pc of the Sun based onMonte Carlomodeling and their average spatial density.The power of the radio emissions of brown dwarfs is roughly constant despite variations in their temperatures.Brown dwarfs may maintain magnetic fields of up to 6kGin strength.Astronomers have estimated brown dwarfmagnetospheresto span an altitude of approximately 10⁷m given properties of their radio emissions.It is unknown whether the radio emissions from brown dwarfs more closely resemble those from planets or stars. Some brown dwarfs emit regular radio pulses, which are sometimes interpreted as radio emission beamed from the poles but may also be beamed from active regions. The regular, periodic reversal of radio wave orientation may indicate that brown dwarf magnetic fields periodically reverse polarity. These reversals may be the result of a brown dwarf magnetic activity cycle, similar to thesolar cycle.

The first brown dwarf of spectral class M found to emit radio waves wasLP 944-20,detected in 2001. The first brown dwarf of spectral class L found to emit radio waves was 2MASS J0036159+182110, detected in 2008. The first brown dwarf of spectral class T found to emit radio waves was2MASS J10475385+2124234.This last discovery was significant since it revealed that brown dwarfs with temperatures similar to exoplanets could host strong >1.7 kG magnetic fields. Although a sensitive search for radio emission from Y dwarfs was conducted at theArecibo Observatoryin 2010, no emission was detected.

Recent developments

A visualization representing a three-dimensional map of brown dwarfs (red dots) that have been discovered within 65 light-years of the Sun

Estimates of brown dwarf populations in the solar neighbourhood suggest that there may be as many as six stars for every brown dwarf.A more recent estimate from 2017 using the young massive star clusterRCW 38concluded that the Milky Way galaxy contains between 25 and 100 billion brown dwarfs.(Compare these numbers to the estimates of the number of stars in the Milky Way; 100 to 400 billion.)

In a study published in Aug 2017NASA'sSpitzer Space Telescopemonitored infrared brightness variations in brown dwarfs caused by cloud cover of variable thickness. The observations revealed large-scale waves propagating in the atmospheres of brown dwarfs (similarly to the atmosphere of Neptune and other Solar System giant planets). These atmospheric waves modulate the thickness of the clouds and propagate with different velocities (probably due to differential rotation).

In August 2020, astronomers discovered 95 brown dwarfs near theSunthrough the project Backyard Worlds: Planet 9.

In 2024 theJames Webb Space Telescopeprovided the most detailed weather report yet on two brown dwarfs, revealing "stormy" conditions. These brown dwarfs, part of aBinary starsystem namedWISE 1049ABdiscovered in 2013, are only 6.5 light-years away from Earth and are the closest brown dwarfs to our sun. Researchers discovered that they have turbulent clouds, likely made of silicate grains, with temperatures ranging from 875 °C (1,607 °F) to 1,026 °C (1,879 °F). This indicates that hot sand is being blown by winds on the brown dwarfs. Additionally, absorption signatures of carbon monoxide, methane, and water vapor were detected.

Binary brown dwarfs

Brown dwarf–brown dwarf binaries

Multi-epoch images of brown dwarf binaries taken with theHubble Space Telescope.The binary Luhman 16 AB (left) is closer to the Solar System than the other examples shown here.

Brown dwarfs binaries of type M, L, and T are less common with a lower mass of the primary.L-dwarfs have a binary fraction of about24+6
−2% and the binary fraction for late T, early Y-dwarfs (T5-Y0) is about 8±6%.

Brown dwarf binaries have a higher companion-to-host ratio$q=M_{B}/M_{A}$for lower mass binaries. Binaries with aM-type staras a primary have for example a broad distribution of q with a preference of q≥0.4. Brown dwarfs on the other hand show a strong preference for q≥0.7. The separation is decreasing with mass: M-type stars have a separation peaking at 3–30astronomical units(au), M-L-type brown dwarfs have a projected separation peaking at 5–8 au and T5–Y0 objects have a projected separation that follows alognormal distributionwith a peak separation of about 2.9 au.

An example is the closest brown dwarf binary Luhman 16 AB with a primary L7.5 dwarf and a separation of 3.5 au and q=0.85. The separation is on the lower end of the expected separation for M-L-type brown dwarfs, but the mass ratio is typical.

It is not known if the same trend continues with Y-dwarfs, because their sample size is so small. The Y+Y dwarf binaries should have a high mass ratio q and a low separation, reaching scales of less than one au.In 2023, the Y+Y dwarfWISE J0336-0143was confirmed as a binary withJWST,with a mass ratio of q=0.62±0.05 and a separation of 0.97 astronomical units. The researchers point out that the sample size of low-mass binary brown dwarfs is too small to determine if WISE J0336-0143 is a typical representative of low-mass binaries or a peculiar system.

Observations of the orbit of binary systems containing brown dwarfs can be used to measure the mass of the brown dwarf. In the case of2MASSW J0746425+2000321,the secondary weighs 6% of the solar mass. This measurement is called a dynamical mass.The brown dwarf system closest to the Solar System is the binary Luhman 16. It was attempted to search for planets around this system with a similar method, but none were found.

Unusual brown dwarf binaries

The wide brown dwarf binarySDSS J1416+1348

The wide binary system2M1101ABwas the first binary with a separation greater than20AU.The discovery of the system gave definitive insights to the formation of brown dwarfs. It was previously thought that wide binary brown dwarfs are not formed or at least are disrupted at ages of 1–10Myr.The existence of this system is also inconsistent with the ejection hypothesis.The ejection hypothesis was a proposed hypothesis in which brown dwarfs form in a multiple system, but are ejected before they gain enough mass to burn hydrogen.

More recently the wide binaryW2150ABwas discovered. It has a similar mass ratio andbinding energyas 2M1101AB, but a greater age and is located in a different region of the galaxy. While 2M1101AB is in a closely crowded region, the binary W2150AB is in a sparsely-separated field. It must have survived any dynamical interactions in its natalstar cluster.The binary belongs also to a few L+T binaries that can be easily resolved by ground-based observatories. The other two areSDSS J1416+13ABand Luhman 16.

There are other interesting binary systems such as theeclipsing binarybrown dwarf system2MASS J05352184–0546085.Photometric studies of this system have revealed that the less massive brown dwarf in the system is hotter than its higher-mass companion.

Brown dwarfs around stars

Main article:Brown-dwarf desert

Brown dwarfs and massive planets in a close orbit (less than 5 au) around stars are rare and this is sometimes described as the brown dwarf desert. Less than 1% of stars with the mass of the sun have a brown dwarf within 3–5 au.

An example for a star–brown dwarf binary is the first discovered T-dwarfGliese 229 B,which orbits around the main-sequence star Gliese 229 A, a red dwarf. Brown dwarfs orbitingsubgiantsare also known, such asTOI-1994bwhich orbits its star every 4.03 days.

There is also disagreement if some low-mass brown dwarfs should be considered planets. TheNASA Exoplanet archiveincludes brown dwarfs with a minimum mass less or equal to 30 Jupiter masses as planets as long as there are other criteria fulfilled (e.g. orbiting a star).The Working Group on Extrasolar Planets (WGESP) of theIAUon the other hand only considers planets with a mass below 13 Jupiter masses.

White dwarf–brown dwarf binaries

LSPM J0241+2553AB,a wide white dwarf(A)–brown dwarf(B) binary.

Brown dwarfs aroundwhite dwarfsare quite rare.GD 165 B,the prototype of the L dwarfs, is one such system.Such systems can be useful in determining the age of the system and the mass of the brown dwarf. Other white dwarf-brown dwarf binaries areCOCONUTS-1 AB(7 billion years old),andLSPM J0055+5948 AB(10 billion years old),SDSS J22255+0016 AB(2 billion years old)WD 0806−661 AB(1.5–2.7 billion years old).

Systems with close,tidally lockedbrown dwarfs orbiting around white dwarfs belong to thepost common envelope binariesor PCEBs. Only eight confirmed PCEBs containing a white dwarf with a brown dwarf companion are known, includingWD 0137-349AB. In the past history of these close white dwarf–brown dwarf binaries, the brown dwarf is engulfed by the star in thered giant phase.Brown dwarfs with a mass lower than 20Jupiter masseswouldevaporateduring the engulfment.The dearth of brown dwarfs orbiting close to white dwarfs can be compared with similar observations of brown dwarfs around main-sequence stars, described as thebrown-dwarf desert.The PCEB might evolve into acataclysmic variable star(CV*) with the brown dwarf as the donor.Simulations have shown that highly evolved CV* are mostly associated with substellar donors (up to 80%).A type of CV*, calledWZ Sge-typedwarf novaoften show donors with a mass near the borderline of low-mass stars and brown dwarfs.The binaryBW Sculptorisis such a dwarf nova with a brown dwarf donor. This brown dwarf likely formed when a donor star lost enough mass to become a brown dwarf. The mass loss comes with a loss of the orbital period until it reaches a minimum of 70–80 minutes at which the period increases again. This gives this evolutionary stage the nameperiod bouncer.There could also exist brown dwarfs that merged with white dwarfs. The novaCK Vulpeculaemight be a result of such a white dwarf–brown dwarf merger.

Formation and evolution

TheHH 1165jet launched by the brown dwarfMayrit 1701117in the outer periphery of thesigma Orioniscluster

Brown dwarfs form similarly to stars and are surrounded byprotoplanetary disks,such asCha 110913−773444.As of 2017 there is only one known proto-brown dwarf that is connected with a largeHerbig–Haro object.This is the brown dwarfMayrit 1701117,which is surrounded by a pseudo-disk and a Keplerian disk.Mayrit 1701117 launches the 0.7-light-year-long jetHH 1165,mostly seen in ionizedsulfur.

Disksaround brown dwarfs have been found to have many of the same features as disks around stars; therefore, it is expected that there will be accretion-formed planets around brown dwarfs.Given the small mass of brown dwarf disks, most planets will be terrestrial planets rather than gas giants.If a giant planet orbits a brown dwarf across our line of sight, then, because they have approximately the same diameter, this would give a large signal fordetection by transit.The accretion zone for planets around a brown dwarf is very close to the brown dwarf itself, so tidal forces would have a strong effect.

Artist's depiction of brown dwarf W1200-7845

In 2020, the closest brown dwarf with an associated primordial disk—WISEA J120037.79-784508.3(W1200-7845)—was discovered by theDisk Detectiveproject when classification volunteers noted its infrared excess. It was vetted and analyzed by the science team who found that W1200-7845 had a 99.8% probability of being a member of the ε Chamaeleontis (ε Cha) youngmoving groupassociation.Its parallax (using Gaia DR2 data) puts it at a distance of 102 parsecs (or 333 lightyears) from Earth—which is within the local Solar neighborhood.

A paper from 2021 studiedcircumstellar discsaround brown dwarfs instellar associationsthat are a few million years old and 140 to 200 parsecs away. The researchers found that these disks are not massive enough to form planets in the future. There is evidence in these disks that might indicate that planet formation begins at earlier stages and that planets are already present in these disks. The evidence for disk evolution includes a decreasing disk mass over time, dust grain growth and dust settling.Disks around brown dwarfs usually have a radius smaller than 40astronomical units,but three disks in the more distantTaurus molecular cloudhave a radius larger than 70 au and were resolved withALMA.These larger disks are able to form rocky planets with a mass >1M_E.There are also brown dwarfs with disks in associations older than a few million years,which might be evidence that disks around brown dwarfs need more time to dissipate. Especially old disks (>20 Myrs) are sometimes calledPeter Pan disks.Currently2MASS J02265658-5327032is the only known brown dwarf that has a Peter Pan disk.

The brown dwarfCha 110913−773444,located 500 light-years away in the constellation Chamaeleon, may be in the process of forming a miniature planetary system. Astronomers fromPennsylvania State Universityhave detected what they believe to be a disk of gas and dust similar to the one hypothesized to have formed the Solar System. Cha 110913−773444 is the smallest brown dwarf found to date (8M_J), and if it formed a planetary system, it would be the smallest-known object to have one.

Planets around brown dwarfs

Artist's impression of a disc of dust and gas around a brown dwarf

According to the IAU working definition (from August 2018) an exoplanet can orbit a brown dwarf. It requires a mass below 13M_Jand a mass ratio of M/M_central<2/(25+√621). This means that an object with a mass up to 3.2M_Jaround a brown dwarf with a mass of 80M_Jis considered a planet. It also means that an object with a mass up to 0.52M_Jaround a brown dwarf with a mass of 13M_Jis considered a planet.

Thesuper-Jupiterplanetary-mass objects2M1207b,2MASS J044144and Oph 98 B that are orbiting brown dwarfs at large orbital distances may have formed bycloud collapserather than accretion and so may besub-brown dwarfsrather thanplanets,which is inferred from relatively large masses and large orbits. The first discovery of a low-mass companion orbiting a brown dwarf (ChaHα8) at a small orbital distance using theradial velocity techniquepaved the way for the detection of planets around brown dwarfs on orbits of a few AU or smaller.However, with a mass ratio between the companion and primary inChaHα8of about 0.3, this system rather resembles a binary star. Then, in 2008, the first planetary-mass companion in a relatively small orbit (MOA-2007-BLG-192Lb) was discovered orbiting a brown dwarf.

Planets around brown dwarfs are likely to becarbon planetsdepleted of water.

A 2017 study, based upon observations withSpitzerestimates that 175 brown dwarfs need to be monitored in order to guarantee (95%) at least one detection of a below earth-sized planet via the transiting method.JWST could potentially detect smaller planets. The orbits of planets and moons in thesolar systemoften align with the orientation of the host star/planet they orbit. Assuming the orbit of a planet is aligned with therotational axisof a brown dwarf orplanetary-mass object,the geometric transit probability of an object similar toIocan be calculated with the formula cos(79.5°)/cos(inclination).The inclination was estimated for several brown dwarfs and planetary-mass objects.SIMP 0136for example has an estimated inclination of 80°±12.Assuming the lower bound of i≥68° for SIMP 0136, this results in a transit probability of ≥48.6% for close-in planets. It is however not known how common close-in planets are around brown dwarfs and they might be more common for lower-mass objects, as disk sizes seem to decrease with mass.

Habitability

Habitability for hypothetical planetsorbitingbrown dwarfs has been studied. Computer models suggesting conditions for these bodies to havehabitable planetsare very stringent, thehabitable zonebeing narrow, close (T dwarf 0.005 AU) and decreasing with time, due to the cooling of the brown dwarf (they fuse for at most 10 million years). The orbits there would have to be of extremely loweccentricity(on the order of 10 to the minus 6) to avoid strongtidal forcesthat would trigger arunaway greenhouse effecton the planets, rendering them uninhabitable. There would also be no moons.

Superlative brown dwarfs

Main article:List of brown dwarfs

In 1984, it was postulated by some astronomers that the Sun may be orbited by an undetected brown dwarf (sometimes referred to asNemesis) that could interact with theOort cloudjust aspassing starscan. However, this hypothesis has fallen out of favor.

Table of firsts

Record	Name	Spectral type	RA/Dec	Constellation	Notes
First discovered	Gliese 569Bab (Companions of M3 field star)	M8.5 and M9	14h54m29.2s+16°06'04 "	Bootes	Imaged in 1985 published in 1988 weighed in 2004
First imaged with coronography	Gliese 229 B	T6.5	06h10m34.62s−21°51'52.1 "	Lepus	Discovered 1994
First withplanemo	2M1207	M8	12h07m33.47s−39°32'54.0 "	Centaurus	Planet discovered in 2004
First with acircumstellar disk	ChaHα1	M7.5	11h07m17.0s−77°35'54 "	Chamaeleon	Disk discovered in 2000, first disk around abona fidebrown dwarf, also first x-ray emitting
First withbipolar outflow	Rho-Oph 102(SIMBAD: [GY92] 102)		16 26 42.758 −24 41 22.24	Ophiuchus	partly resolved outflow
First with large-scaleHerbig-Haro object	Mayrit 1701117 (Herbig-Haro object:HH 1165)	proto-BD	05 40 25.799 −02 48 55.42	Orion	projected length of the Herbig-Haro object: 0.8light-years(0.26pc)
First field type (solitary)	Teide 1	M8	3h47m18.0s+24°22'31 "	Taurus	1995
First as a companion to a normal star	Gliese 229 B	T6.5	06h10m34.62s−21°51'52.1 "	Lepus	1995
First spectroscopic binary brown dwarf	PPL 15 A, B	M6.5	03h48m4.659s+23° 39' 30.32″	Taurus	Basri and Martín 1999
First eclipsing binary brown dwarf	2M0535-05	M6.5		Orion	Stassun 2006, 2007 (distance ~450 pc)
First binary brown dwarf of T Type	Epsilon Indi Ba, Bb	T1 + T6	22h03m21.65363s−56° 47′ 09.5228″	Indus	Distance: 3.626pc
First binary brown dwarf of Y Type	WISE J0336−0143	Y+Y	03h36m05.052s−01° 43′ 50.48″	Eridanus	2023
First trinary brown dwarf	DENIS-P J020529.0-115925A/B/C	L5, L8 and T0	02h05m29.40s−11°59'29.7 "	Cetus	Delfosse et al. 1997
First halo brown dwarf	2MASS J05325346+8246465	sdL7	05h32m53.46s+82°46'46.5 "	Gemini	Burgasser et al. 2003
First with late-M spectrum	Teide 1	M8	3h47m18.0s+24°22'31 "	Taurus	1995
First with L spectrum	GD 165B	L4	14h24m39.144s09° 17′ 13.98″	Boötes	1988
First with T spectrum	Gliese 229B	T6.5	06h10m34.62s−21°51'52.1 "	Lepus	1995
Latest-T spectrum	ULAS J003402.77−005206.7	T9		Cetus	2007
First with Y spectrum	CFBDS0059	~Y0	00h59m10.83s−01° 14′ 01.3″	Cetus	2008; this is also classified as a T9 dwarf, due to its close resemblance to other T dwarfs.
First X-ray-emitting	ChaHα1	M8		Chamaeleon	1998
First X-ray flare	LP 944–20	M9V	03h39m35.22s−35°25'44.1 "	Fornax	1999
First radio emission (in flare and quiescence)	LP 944-20	M9V	03h39m35.22s−35°25'44.1 "	Fornax	2000
First potential brown dwarf auroras discovered	LSR J1835+3259	M8.5		Lyra	2015
First detection of differential rotation in a brown dwarf	TVLM 513-46546	M9	15h01m08.3s+22°50'02 "	Boötes	Equator rotates faster than poles by 0.022 radians / day
First confirmed brown dwarf to have survived the primary'sred giantphase	WD 0137−349B	L8	01h39m42.847s−34° 42′ 39.32″	Sculptor (constellation)

Table of extremes

Record	Name	Spectral type	RA/Dec	Constellation	Notes
Oldest	LSPM J0055+5948 B Wolf 1130 C CWISE J0602-4624	T8 sdT8 L8	00h55m58.300s+59° 48′ 02.53″ or 20h05m02.1951s+54° 26′ 03.234″ or 06h02m02.17s−46° 24′ 47.8″	Cassiopeia,CygnusorPictor	three of the few examples with a good age estimate: LSPM J0055B: 10±3billion years Wolf 1130C: >10 billion years CWISE J0602-4624:10.9+2.6 −2.0billion years
Youngest	2MASS J05413280-0151272	M8.5	05h41m32.801s−01° 51′ 27.20″	Orion	One brown dwarf member of the about 0.5 Myr-oldFlame Nebula.20.9MJobject
Most massive	SDSS J010448.46+153501.8	usdL1.5	01h04m48.46s+15°35'01.8 "	Pisces	distance is ~180–290 pc, mass is ~88.5–91.7MJ.Transitional brown dwarfs.
Metal-rich
Metal-poor	SDSS J010448.46+153501.8	usdL1.5	01h04m48.46s+15°35'01.8 "	Pisces	distance is ~180–290 pc, metallicity is ~0.004ZSol.Transitional brown dwarfs.
Least massive
Largest
Smallest	WISEA 1810−1010	esdT	18h10m06.18s−10° 10′ 00.5″	Serpens	Radius is0.65+0.31 −0.19RJ(~92,400 km)
Fastest rotating	2MASS J03480772−6022270	T7	03h48m07.72s–60°22'27.1 "	Reticulum	Rotational period of1.080+0.004 −0.005hours
Farthest	KMT-2016-BLG-2142 b		17h52m27.0s–29° 23′ 04″	Sagittarius	(microlensing)has a distance of 5,850 to 8,020 parsec. Could also be massive gas giant.
Nearest	Luhman 16AB	L7.5 + T0.5 ± 1	10h49m18.723s−53° 19′ 09.86″	Vela	Distance: ~6.5 ly
Brightest	LP 944-20	opt: M9beta, IR: L0:	03h39m35.220s−35° 25′ 44.09″	Fornax	According to the ultracool fundamental propertiesthis object shows signs of youth and could therefore be a brown dwarf with 19.85±13.02MJand JMKO=10.68±0.03 mag
Dimmest	L 97-3B	Y1	08h06m53.736s−66° 18′ 16.74″	Volans	jmag=25.42, planetary-mass object
Hottest
Coolest	WISE 0855−0714	Y4	08h55m10.83s−07° 14′ 42.5″	Hydra	Temperature: −48 to −13 °C (225 to 260 K; −54 to 9 °F)
Coolest radio-flaring	WISE J062309.94-045624.6	T8	06h23m09.28s−04°56'22.8 "	Monoceros	699 K (426 °C; 799 °F) brown dwarf with 4.17 mJy bursts
Most dense	TOI-569b		07h40m24.658s−42° 09′ 16.74″	Puppis	Transiting,has64.1MJwith a diameter 0.79 ± 0.02 times that of Jupiter. Density is 171.3g/cm3.
Least dense

Gallery

• 
  Brown dwarf illustration

See also

• Fusor (astronomy)
• Brown-dwarf desert– Theorized range of orbits around a star within which brown dwarfs cannot exist as companion objects
• Blue dwarf (red-dwarf stage)– Hypothetical class of star that develops from a red dwarf
• Dark matter– Concept in cosmology
• Exoplanet– Planet outside the Solar System
• Stellification
• WD 0032-317 b
• List of brown dwarfs
• List of Y-dwarfs

External links

• HubbleSite newscenter – Weather patterns on a brown dwarf
• Allard, France; Homeier, Derek (2007)."Brown dwarfs".Scholarpedia.2(12): 4475.Bibcode:2007SchpJ...2.4475A.doi:10.4249/scholarpedia.4475.

History

• Kumar, Shiv S.;Low-Luminosity Stars.Gordon and Breach, London, 1969—an early overview paper on brown dwarfs
• The Columbia Encyclopedia: "Brown Dwarfs"

Details

• A current list of L and T dwarfs
• A geological definition of brown dwarfs,contrasted with stars and planets (via Berkeley)
• I. Neill Reid's pages at theSpace Telescope Science Institute:
  • On spectral analysisofM dwarfs,L dwarfs,andT dwarfs
  • Temperature and mass characteristicsof low-temperature dwarfs
• First X-ray from brown dwarf observed,Spaceref, 2000
• Montes, David;"Brown Dwarfs and ultracool dwarfs (late-M, L, T)",UCM
• Wild Weather: Iron Rain on Failed Stars—scientists are investigating astonishing weather patterns on brown dwarfs, Space, 2006
• NASA Brown dwarf detectivesArchived2014-10-17 at theWayback Machine—Detailed information in a simplified sense
• Brown Dwarfs—Website with general information about brown dwarfs (has many detailed and colorful artist's impressions)

Stars

• Cha H Alpha 1stats and history
• "A census of observed brown dwarfs"(not all confirmed), 1998
• Luhman, Kevin L.; Adame, Lucía; d'Alessio, Paola; Calvet, Nuria; Hartmann, Lee; Megeath, S. Thomas; Fazio, Giovanni G. (2005). "Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk".The Astrophysical Journal.635(1): L93–L96.arXiv:astro-ph/0511807.Bibcode:2005ApJ...635L..93L.doi:10.1086/498868.S2CID11685964.
• Michaud, Peter; Heyer, Inge; Leggett, Sandy K.; and Adamson, Andy; "Discovery Narrows the Gap Between Planets and Brown Dwarfs", Gemini and Joint Astronomy Centre, 2007
• Deacon, N. R.; Hambly, N. C. (2006)."The possiblity of detection of ultracool dwarfs with the UKIRT Infrared Deep Sky Survey".Monthly Notices of the Royal Astronomical Society.371(4): 1722–1730.arXiv:astro-ph/0607305.Bibcode:2006MNRAS.371.1722D.doi:10.1111/j.1365-2966.2006.10795.x.